I guess part of the issue here is that cars are a kind of coordination problem - if nobody else is using a car on the roads, cars are quite a good choice. They are fast and easy, and go where you want.

However, each person who uses a car creates some external costs (a high traffic factor / unit of person-flow) which are borne by all the other road users. Once people choosing the "defect" strategy (cars) have enough numbers, the "cooperate" strategy (not cars) gets broken for other solutions that use the road network, so the system fails to a bad equilibrium if there are enough defectors in the population.